To elucidate the particle growth mechanism in propylene polymerization with high-yield MgCl2-supported Ziegler-Natta catalysts, observations have been carried out by electron microscopy on a series of samples having different degrees of polymer growth (from 0.1 to 1000 g/g of catalyst). Topics such as surface and bulk morphology, catalyst fragmentation, as well as distribution of the catalyst residues in the polymer have been investigated. The experimental data suggest that if the site distribution in the catalyst is uniform and the polymerization conditions are mild, the polymer growth starts uniformly throughout the catalyst particle, which then undergoes an even and progressive fragmentation into very fine units homogeneously dispersed in the polymer matrix. The above results thus provide further experimental support to the particle growth mechanism outlined in the multigrain or polymeric flow models.